Vehicle internal combustion engines may incorporate turbochargers to boost engine torque output. For example, engines which have been downsized for improved fuel economy may require boost to compensate for the loss in engine power resulting from the downsizing, and may obtain such boost via a turbocharger (e.g., the turbocharger may provide boost by creating forced induction of intake air and thereby improving engine torque output). However, various disadvantages may be associated with using a turbocharger alone. In the case of downsized engines, incorporation of a turbocharger may limit the extent the engine can be downsized as it may not be possible to provide a desired pressure ratio or transient behavior over the full operating range of the engine with a turbocharger alone. Further, more generally, turbochargers may have a slow transient response, which may result in undesirable lag in engine torque output.
Furthermore, when exhaust flowing from multiple engine cylinders is directed to an inlet of the turbocharger turbine, undesirable cross-talk may between the cylinders depending on the firing order and cam timing of the cylinders, which may interfere with combustion. As one example of undesirable cylinder cross-talk in a turbocharged engine, a turbocharged four-cylinder engine may have a 1-3-4-2 firing order, where cylinders 1 and 4 are outer cylinders of the engine block and cylinders 2 and 3 are inner cylinders of the engine block. Communication between the outer cylinders and inner cylinders may occur upstream of the turbine of the turbocharger. For example, late in the exhaust blow-down event for cylinder 1, cylinder 3 may start to blow down which may increase the pressure and temperature of gas trapped in cylinder 1 and may increase residual content in that cylinder. As a result, combustion may be negatively affected, there may be an increased propensity for knock, and more boost may be required to achieve a desired engine power output. These issues may ultimately limit the power capability of the engine.
Various approaches may be used to address the above issues. To address turbocharger lag, some approaches incorporate a supercharger to provide boost along with the turbocharger, where one or both of the devices may be used at a given time. For example, US 2010/0263375 describes a boost system comprising a turbocharger and a supercharger operable as either a compressor or an expander. At lower engine speeds, the supercharger provides boost, and at higher engine speeds (e.g., relative to the lower engine speeds), the turbocharger provides boost while the supercharger operates as an expander to provide cooling. In this approach, exhaust from all engine cylinders is directed through the turbocharger turbine. As such, this approach may suffer from cylinder cross-talk, which may negatively affect engine combustion.
To address cylinder cross-talk, some approaches may incorporate a twin-scroll turbocharger. Twin-scroll turbochargers address cylinder cross-talk by providing two exhaust paths for cylinders whose exhaust pulses may interfere with each other, each path leading to a different turbine. For example, in a four-cylinder engine with a firing sequence of 1-3-4-2, exhaust valve overlap may occur between cylinders 1 and 2 during the expansion stroke of cylinder 1. However, because the twin-scroll turbocharger provides separate exhaust paths for cylinders 1 and 2, the exhaust pulse from cylinder does not interfere with combustion in cylinder 2. Further, because such valve overlap does not have a negative effect on combustion, cam duration may be increased, thereby decreasing engine pumping work and increasing fuel efficiency. Despite these advantages, the inventors have identified various issues with the twin-scroll turbocharger approach. Twin-scroll turbochargers may be relatively expensive and may have low durability due to their complex structure relative to a single turbocharger. Further, their functionality may be comprised at high temperatures.
The inventors herein have recognized that incorporating a supercharger in an engine system to address transient response issues or replacing a turbocharger with a twin-scroll turbocharger to address cylinder cross-talk issues may not produce satisfactory results, and may even introduce further issues such as those described above. In contrast with the above approaches, the inventors herein have identified an engine system and associated methods for providing boost via one or both of a turbocharger and supercharger, thereby avoiding a slow transient response, where exhaust from only a subset of engine cylinders flow through the turbocharger turbine so as to avoid cylinder cross-talk. In accordance with one example method, exhaust from only a first subset of cylinders may be directed to a turbine of a turbocharger via an IEM, exhaust from only a second subset of cylinders may be directed to bypass the turbine via the IEM, and intake air may be compressed via one or both of a supercharger and a compressor of the turbocharger. In this way, by only directing exhaust from a subset of engine cylinders through the turbine, communication will not occur between cylinders of the different subsets even when exhaust valve openings of the cylinders overlap (which may occur depending on the firing sequence of the cylinders). Accordingly, cam duration may be increased, thereby decreasing pumping work and increasing fuel efficiency. Because exhaust from the second subset of cylinders does not flow through the turbine, this exhaust may flow through a close-coupled catalyst, thereby reducing catalyst light-off times which may improve engine emissions. Also, because the use of an IEM may be compatible with this engine system, turbocharger housing material costs may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.